Drosophila is a model system for studying how steroid hormones regulate neuronal development. The steroid hormones, the ecdysones, cause pruning of larval dendrites followed by sprouting of the adult arbor. Ecdysone acts through 3 isoforms of the ecdysone receptor (EcR) and isoform blends shape the neuronal response. Remodeling sensory neurons that can be viewed through the transparent cuticle provide an unparalleled opportunity to follow remodeling of complex dendritic trees in a living animal. Two-photon imaging used with molecular genetics will make it possible to link isoform specific patterns of gene activation to discrete cytodynamic modules and finally to the overall shape of the mature arbor. The specific aims are to: 1) Use 2-photon imaging of membrane and cytoskeleton dynamics to define events underlying dendritic arbor collapse during stereotyped pruning. Studies focus on filopodial dynamics, how they relate to sites of arbor destabilization, and whether destabilization propagates within an arbor. Loss and gain of function studies will define the role of different EcR isoforms in these processes. 2) Determine the role of the epidermis in dendritic collapse. Dominant negative EcRs targeted to the neuron and/or the epidermis will block responses to steroid and the impact on distal retraction and arbor collapse will be determined. The role of filopodia in receiving epidermal cues will be examined. 3) Assess the role of phagocytes in dendrite removal. We will use time-lapse analysis of phagocyte responses to destabilized dendritic arbor, and determine the impact of blocking phagocytic activity on arbor removal. 4) Test the hypothesis that branch lability underlies growth plasticity and that the degree of plasticity is imposed by the levels of EcR-B. Loss and gain of function approaches will alter EcR isoform ratios and 2-photon microscopy will follow the cytodynamics of outgrowth. 5) Use EcR dominant-negative mutations to selectively suppress cytodynamic modules to establish the role of each on growth dynamics and on the final form of the arbor.